Decorative optical displays have been popular as novelty items for over a century. Most of the known decorative optical displays (also referred to as "light sculptures," or "decorative lighting assemblies,") use various combinations of mirrors, light bulbs, lasers, prisms, lenses, colored glass, and diffraction gratings to achieve a variety of aesthetically pleasing visual effects.
A typical example of a decorative optical display apparatus is U.S. Pat. No. 5,276,599 to Neely, which discloses a light sculpture which displays light patterns inside a box-like enclosure. The enclosure includes planar mirrored panels joined together at right angles to form reflective walls, with one wall being translucent to permit an observer to look inside. Attached to the bottom of the enclosure is a hollow base which includes a plurality of light sources. The light patterns produced consist of light emanating directly from the light sources, as well as light reflected from the walls of the enclosure.
An example of an optical display apparatus that employs a diffraction grating is U.S. Pat. No. 734,133 to Porter (hereinafter, "the '133 patent"). The '133 patent discloses a light source, such as an arc lamp, placed behind a flat transmissive diffraction grating. The light source is then viewed through the diffraction grating by an observer. The diffraction grating disperses the light in a direction perpendicular to the orientation of the grating lines, so that an observer sees a "spoke" of light varying in color over its length. For a diffraction grating having multiple sets of grating lines (so called "multi-axis" diffraction gratings), multiple spokes of light are produced equal to the number of superimposed grating lines. Also, two separate diffraction gratings in proximity to one another can be used and motion can be imparted to the light patterns by rotating the diffraction gratings.
While the optical display of the '133 patent is capable of creating aesthetically pleasing light patterns, it also has several major drawbacks. The first is that placing one or more light sources directly behind the transmissive diffraction grating makes for a display apparatus that is non-compact. The second is that achieving motion in the light pattern by rotating the diffraction gratings makes the display apparatus complicated and expensive. The third is that it is not always desirable to view undiffracted light through the transmissive diffraction grating because the undiffracted light tends to overwhelm the diffracted light.